Patent application title: GEOPHONE HAVING IMPROVED SENSITIVITY

Abstract:

A geophone utilizing an Alnico-9 magnet and having an improved sensitivity
over Alnico-9 geophones of prior art through the lengthening of the
parasitic air gap between the upper and lower pole pieces which, results
in less magnetic flux leakage. The flux concentration through the
geophone coils is increased and shifted towards the ends of the magnet.
The increase of sensitivity of geophone of the present invention over
prior art geophones may exceed 3 dB. The axial length of the coil bobbin
is increased, and the positions of the electrical coils are moved towards
the ends of the magnet to align with the shifted magnetic flux.

Claims:

1. A geophone (110) comprising:a cylindrical magnet (114) having a
length-to-diameter ratio (lm/dm) greater than 0.5 and less than
1.0;upper and lower pole piece caps (116, 118) receiving the upper and
lower ends of said magnet, respectively, said upper and lower pole piece
caps (116, 118) together defining an overall longitudinal pole-to-pole
length (lpp) and a parasitic air gap (125) therebetween having a
longitudinal air gap length (lg), said upper and lower pole piece
caps (116, 118) having a parasitic gap length to pole-to-pole length
ratio (lg/lpp) greater than 0.25;a tubular outer cylinder
housing (120), said magnet (114) and upper and lower pole piece caps
(116, 118) being coaxially received within and connected to said housing
(120); andan electrical coil (140, 142) disposed between said magnet
(114) and said housing (120) and movable in an axial direction with
respect to said housing (120).

2. The geophone (110) of claim 1 wherein:said magnet (114) is made of
Alnico-9 material.

3. The geophone (110) of claim 1 wherein:said length to diameter ratio
(lm/dm) of said magnet (114) is greater than 0.7 and less than
0.9.

6. The geophone (110) of claim 5 wherein:said upper and lower pole piece
caps (116, 118) each have a pole length to lip length ratio
(lp/ll) greater than 0.4 and less than 1.0.

7. A geophone (110) comprising:a cylindrical magnet (114) having a
length-to-diameter ratio (lm/dm) greater than 0.5 and less than
1.0;upper and lower pole piece (116, 118) caps receiving the upper and
lower ends of said magnet (114), respectively, said upper and lower pole
piece caps (116, 118) together defining a parasitic air gap (125)
therebetween having a longitudinal parasitic gap (125) length (lg);a
tubular outer cylindrical housing (120), said magnet (114) and upper and
lower pole piece caps (116, 118) being coaxially received within and
connected to said housing (120), said housing (120) and said upper and
lower pole piece caps (116, 118) together defining upper and lower
annular air gaps (122, 124), respectively, each of said upper and lower
annulus air gaps (122, 124) having a radial air gap dimension (ra),
a parasitic gap length to radial air gap ratio (lg/ra) being
greater than 4.0; andan electrical coil (140, 142) assembly disposed in
said upper and lower annular air gaps (122, 124) and movable in an axial
direction with respect to said housing (120).

8. The geophone (110) of claim 7 wherein:said magnet (114) is made of
Alnico-9 material.

9. The geophone (110) of claim 7 wherein:said length to diameter ratio
(lm/dm) of said magnet (114) is greater than 0.7 and less than
0.9.

10. The geophone (110) of claim 7 wherein:said parasitic gap to radial air
gap ratio (lg/ra) is greater than 6.0.

11. The geophone (110) of claim 7 wherein:said housing (120) has a
pole-to-pole region that longitudinally aligns with said magnet (114) and
said upper and lower pole caps (116, 118); andsaid pole-to-pole region of
said housing (120) has a wall-thickness to radial air gap ratio
(th/ra) that is greater than 0.7.

12. The geophone (110) of claim 11 wherein:said pole-to-pole-region of
said housing (120) has a wall-thickness to radial air gap ratio
(th/ra) that is greater than 1.0.

13. A geophone (110) comprising:an Alnico-9 cylindrical magnet (114);upper
and lower pole piece caps (116, 118) receiving the upper and lower ends
of said magnet (114), respectively, said upper and lower pole piece caps
(116, 118) together defining a parasitic air gap (125) therebetween
having a longitudinal parasitic gap length (lg), said upper and
lower pole piece caps (116, 118) each defining a longitudinal pole length
(lp) and a lip thickness (tl);a tubular outer cylindrical
housing (120), said magnet (114) and said upper and lower pole piece caps
(116, 118) being coaxially received within and connected to said housing
(120), said housing (120) and said upper and lower pole piece caps (116,
118) together defining upper and lower annular air gaps (122, 124),
respectively, each of said upper and lower annular air gaps (122, 124)
having a radial air gap dimension (ra); andan electrical coil
assembly (140, 142) disposed in said upper and lower annular air gaps
(122, 124) and movable in an axial direction with respect to said housing
(120);wherein said geophone (110) is arranged so that the relation l p
l g r a t l ##EQU00006## is greater than 14.

14. The geophone (110) of claim 13 wherein:said geophone (110) is arranged
so that the relation of l p l g r a t l ##EQU00007## is
greater than 20.

15. The geophone (110) of claim 13 wherein:said magnet (114) has a length
to diameter ratio (lm/dm) greater than 0.5 and less than 1.0.

21. A geophone magnetic field subassembly comprising:a cylindrical magnet
(114) having a length-to-diameter ratio (lm/dm) greater than
0.5 and less than 1.0; andupper and lower pole piece caps (116, 118)
receiving the upper and lower ends of said magnet, respectively, said
upper and lower pole piece caps (116, 118) together defining an overall
longitudinal pole-to-pole length (lpp) and a parasitic air gap (125)
therebetween having a longitudinal air gap length (lg), said upper
and lower pole piece caps (116, 118) having a parasitic gap length to
pole-to-pole length ratio (lg/lpp) greater than 0.25.

22. A geophone (110) comprising:an Alnico-9 cylindrical magnet (114)
defining an axial magnet length (lm);upper and lower pole piece
(116, 118) caps receiving the upper and lower ends of said magnet (114),
respectively, said upper and lower pole piece caps (116, 118) defining a
pole thickness (tp) such that a ratio of said pole thickness to
magnet length (tp/lm) is greater than 0.15;a tubular outer
cylindrical housing (120), said magnet (114) and upper and lower pole
piece caps (116, 118) being coaxially received within and connected to
said housing (120), said housing (120) and said upper and lower pole
piece caps (116, 118) together defining upper and lower annular air gaps
(122, 124), respectively; andan electrical coil (140, 142) assembly
disposed in said upper and lower annular air gaps (122, 124) and movable
in an axial direction with respect to said housing (120).

23. A geophone (110) comprising:a cylindrical magnet (114);upper and lower
pole piece caps (116, 118) receiving the upper and lower ends of said
magnet, respectively, said upper and lower pole piece caps (116, 118)
together defining a parasitic air gap (125) therebetween having a
longitudinal air gap length (lg);a tubular outer cylindrical housing
(120), said magnet (114) and upper and lower pole piece caps (116, 118)
being coaxially received within and connected to said housing (120), said
housing (120) and said upper and lower pole piece caps (116, 118)
together defining upper and lower annular air gaps (122, 124),
respectively, each of said upper and lower annulus air gaps (122, 124)
having a radial air gap dimension (ra) such that a ratio of said air
gap length to said radial air gap dimension (lg/ra) is greater
than 5.7; andan electrical coil (140, 142) assembly disposed in said
upper and lower annular air gaps (122, 124) and movable in an axial
direction with respect to said housing (120).

24. A geophone (110) comprising:a cylindrical magnet (114);upper and lower
pole piece caps (116, 118) receiving the upper and lower ends of said
magnet, respectively;a tubular outer cylindrical housing (120), said
magnet (114) and upper and lower pole piece caps (116, 118) being
coaxially received within and connected to said housing (120), said
housing (120) and said upper and lower pole piece caps (116, 118)
together defining upper and lower annular air gaps (122, 124),
respectively, each of said upper and lower annulus air gaps (122, 124)
having a radial air gap dimension (ra), the region of said housing
between said upper and lower annulus air gaps (122, 124) defining a
housing wall thickness dimension (th) such that a ratio of said
housing wall thickness dimension to said radial air gap dimension
(th/ra) is greater than 0.7; andan electrical coil (140, 142)
assembly disposed in said upper and lower annular air gaps (122, 124) and
movable in an axial direction with respect to said housing (120).

25. A geophone (110) comprising:a cylindrical magnet (114);upper and lower
pole piece caps (116, 118) receiving the upper and lower ends of said
magnet, respectively, said upper and lower pole piece caps (116, 118)
defining a pole length (lp) and a lip length (ll) such that a
ratio of said pole length to said lip length (lp/lp) is greater
than 2.1 and less than 5.0;a tubular outer cylindrical housing (120),
said magnet (114) and upper and lower pole piece caps (116, 118) being
coaxially received within and connected to said housing (120), said
housing (120) and said upper and lower pole piece caps (116, 118)
together defining upper and lower annular air gaps (122, 124),
respectively; andan electrical coil (140, 142) assembly disposed in said
upper and lower annular air gaps (122, 124) and movable in an axial
direction with respect to said housing (120).

26. A geophone (110) comprising:a cylindrical magnet (114) having a
length-to-diameter ratio (lm/dm) greater than 0.5 and less than
1.0;upper and lower pole piece caps (116, 118) receiving the upper and
lower ends of said magnet, respectively, said upper and lower pole piece
caps (116, 118) defining a pole length (lp) and a lip length
(ll) such that a ratio of said pole length to said lip length
(lp/lp) is greater than 1.7;a tubular outer cylindrical housing
(120), said magnet (114) and upper and lower pole piece caps (116, 118)
being coaxially received within and connected to said housing (120), said
housing (120) and said upper and lower pole piece caps (116, 118)
together defining upper and lower annular air gaps (122, 124),
respectively; andan electrical coil (140, 142) assembly disposed in said
upper and lower annular air gaps (122, 124) and movable in an axial
direction with respect to said housing (120).

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]This invention relates generally to geophones used in seismic
exploration.

[0003]2. Description of the Prior Art

[0004]FIG. 1 shows a vertical geophone 10 of conventional design. FIG. 1
is a cross section taken along the longitudinal axis 12 of the geophone
10. Geophone 10 employs a cylindrical magnet 14, cylindrical upper and
lower ferrous pole pieces 16, 18, and a tubular ferrous outer housing 20,
which together form a magnetic circuit.

[0005]Upper and lower pole pieces 16, 18 each have a cap-like shape so
that they fit over and receive the upper and lower ends of magnet 14,
respectively. The tubular portion of the upper and lower pole pieces that
enclose the sides of cylindrical magnet 14 are referred to herein as the
pole piece lips 80, 82. Magnet 14 and pole pieces 16, 18 are received
within outer cylindrical housing 20. An upper annular air gap 22 exists
between upper pole piece 16 and outer housing 20, and a lower annular air
gap 24 exists between lower pole piece 18 and outer housing 20.

[0006]Lower pole piece 18 and the lower end of outer housing 20 are
connected to a lower end cap 26, which is in turn connected to a stake
(not shown) that is placed within the ground to couple ground vibrations
to the magnet and pole pieces. Lower end cap 26 is typically formed of a
dielectric plastic material. An upper end cap 28 is connected between
upper pole piece 16 and the upper end of outer housing 20. Upper end cap
28 is also typically made of a dielectric plastic material.

[0007]Within the annular space formed between magnet 14 and upper and
lower pole pieces 16, 18, on the one hand, and cylindrical outer housing
20 on the other, an inertial member--generally a cylindrical bobbin
30--is suspended between an upper frequency-tuned spring 32 and a lower
frequency-tuned spring 34. Upper frequency spring 32 is carried by a thin
dielectric wafer 52, which in turn is carried by the upper pole piece 16.
Lower frequency spring 34 is carried by a contact spring 36, which in
turn is carried by lower end cap 26. The frequency springs allow the
magnet 14, pole pieces 16, 18, and outer housing 20 to vibrate up and
down axially with respect to bobbin 30 while the bobbin remains
essentially motionless and decoupled from the rest of the geophone. The
frequency springs are designed and tuned to provide a desired resonant
frequency.

[0008]An upper electrical coil 40 is wound about bobbin 30 in the vicinity
of the upper air gap 22, and a lower electrical coil 42 is wound about
bobbin 30 in the vicinity of lower air gap 24. The winding direction of
upper coil 40 is opposite to the winding direction of lower coil 42. An
electrical circuit is formed as follows: The upper lead 80 of upper coil
40 is connected to the outer circumference of upper frequency spring 32
by solder joint. The inner circumference of the upper frequency spring
makes sliding electrical contact with a first lead 60 that passes through
upper end cap 28. The inner circumference of the upper frequency spring
is electrically isolated from upper pole piece 16 by thin dielectric
wafer 52 that is positioned therebetween. The lower lead of upper coil 40
is connected to the upper lead of lower coil 42 by a connecting wire 62.
The lower lead 82 of lower coil 42 is connected to the outer
circumference of lower frequency spring 34 by solder joint. The inner
circumference of lower frequency spring 34 makes sliding electrical
contact with the lower surface of lower pole piece 18. Contact spring 36
forces the inner circumference of lower frequency spring 34 to abut lower
pole piece 18 in opposition to the force of gravity. An electrical path
is formed between lower pole piece 18 and upper pole piece 16 through
abutting contact of the upper and lower pole pieces with magnet 14.
Finally, upper pole piece 16 makes sliding electrical contact with a
second lead 64 that passes through upper end cap 28. The first and second
leads 60, 64 are connected to geophone recording circuitry through a
seismic cable. The arrangement of this electrical circuit allows bobbin
30 to freely rotate within geophone 10, thus minimizing the possibility
of damage from rough handling.

[0009]Geophone 10 defines a magnetic circuit as follows: A magnetic flux
is created by and passes axially through magnet 14. This magnetic flux is
channeled through the upper and lower pole pieces 16, 18, passes radially
through upper and lower air gaps 22, 24, and then passes through outer
cylindrical housing 20 to form a complete magnetic circuit. The complete
magnetic circuit is illustrated via flux line 71 of FIG. 2.

[0010]In operation, a terrestrial vibration causes the magnetic circuit
components, and hence the magnetic flux, to vibrate up and down relative
to bobbin 30, which remains essentially inertially stationary. As the
radial flux lines cut the upper and lower coils 40, 42, an electromotive
force is induced in the coils according to Faraday's law. This induced
voltage is measured at the first and second leads 60, 64 via the
electrical circuit described above.

[0011]FIG. 2 is a cut away view in partial cross-section of a portion of
prior art geophone 10, shown without bobbin 30 and coils 40, 42 for
simplicity. Radial lines of magnetic flux 70 crossing air gaps 22, 24
between upper and lower pole pieces 16, 18 and outer cylinder housing 20
are illustrated. Although the radial air gap magnetic flux 70 is normal
to the axis of magnet 14, the flux has a tendency to fringe across the
air gaps 22, 24 at the upper and lower ends of the pole piece lips 80,
82, as depicted by the bulging flux lines 72. The effect of the fringing
is to increase the cross-sectional area and thus the permeance of the
high-reluctance air gap. This fringing effect creates non-linearities in
the magnetic flux density within the air gap, which results harmonic
distortion and a non-linear geophone response. Thus it has heretofore
been a concern of the prior art to maximize the linearity of the magnetic
flux density passing through upper and lower air gaps to minimize
harmonic distortion induced in the geophone response. Geophone 10 of
prior art is designed to maximize linearity by having a long length lp of
the upper and lower pole pieces 16, 18, so that the cross-sectional area
of the air gaps is increased and the fringing of the flux is lowered. In
order to keep the size and weight of the geophone minimal, the pole piece
lips 80, 82 are lengthened to concentrate the magnetic flux near the
center of magnet 14.

[0012]Some of the magnetic flux will also leak across the air gap 25
formed between the upper and lower pole pieces 14, 16. Because this flux
leakage does not pass through the upper and lower coils 22, 24, it does
not contribute to signal generation, and is thus referred to as a
parasitic flux leakage. This parasitic flux leakage is shown by flux
lines 74 in FIG. 2. Although increasing the lip length ll of the
upper and lower pole pieces increases geophone response linearity, it
also has the effect of decreasing the length lg of the parasitic air
gap 25. This smaller lg results in lower parasitic reluctance,
greater parasitic flux leakage, and thus a concomitant reduction in
geophone sensitivity.

[0013]In conducting a seismic survey, multiple geophone channels are
recorded. Because geophone sensitivity is low, each geophone channel
typically includes between six and twelve geophones in order to produce a
required voltage signal for recording. As computing power increases, it
has become more desirable to conduct high resolution surveys across large
geographical areas, which necessitates that large number of geophone
channels are employed in a given survey. Therefore, it is likewise
desirable to increase geophone sensitivity so that a fewer number of
geophones are required per channel to obtain a sufficient signal
strength, thus reducing the overall capital and operational cost of the
survey system.

[0014]Damping of bobbin 30 is necessary so that there will not be
continual oscillation of bobbin relative to the rest of the geophone.
Referring to prior art geophone 10 of FIG. 1, damping of bobbin 30 is a
function of the mass and the electrical conductivity of bobbin 30 (the
conductivity affects the formation of eddy currents formed in bobbin 30
by Faraday induction, which eddy currents flowing in a magnetic field
result in a force being exerted on bobbin 30 that opposes the motion that
created the eddy currents). There is limited ability to control the
conductivity of bobbin 30, and machining tolerances prohibit fine control
of the mass of bobbin 30. Once a graphic design is finalized, the mass of
upper and lower coils 40, 42 is fixed. The result of these factors is an
inability to tightly control the damping tolerance. It is therefore
desirable to control the bobbin mass more tightly in order to more
precisely control the geophone damping.

[0015]Referring to prior art vertical geophone 10 of FIG. 1, the lower
lead of lower coil 42 is electrically connected to lower pole piece 16 by
lower frequency spring 34. Typically, the coil lead is soldered to the
outer circumference of lower frequency spring 34. The inner circumference
of the lower frequency spring makes a sliding electrical contact with the
lower surface of lower pole piece 16, so that lower frequency spring 34
is free to rotate with respect to the lower pole piece 16.

[0016]In order to keep lower frequency spring 34 seated against lower pole
piece 16 for electrical continuity, a contact spring 36 is placed between
lower end cap 26 and lower frequency spring 34, which puts an upward
compressive force on the inner circumference of lower frequency spring
34. However, because lower frequency spring 34 is supported by a
resilient contact spring 36, rather than a rigid, stable platform,
distortion of the natural sinusoidal response to an impulse is created.
Moreover, tuning the geophone frequency response by control of the lower
frequency spring 34 is made more difficult because of the serial
spring-spring arrangement.

[0017]Other geophone designs of prior art, such as that disclosed in U.S.
Pat. No. 5,119,345 issued to Woo et al., seat the lower frequency spring
directly on the lower end cap. However, these design do not employ the
lower frequency spring as an electrical circuit element. For example, in
the Woo '345 patent, two upper pigtail springs 40 and 42 are used to
provide electrical connections between the geophone coils and the
geophone case. Thus, the bobbin and coil assembly have a limited ability
to rotate within the geophone housing, which can result in damage to the
geophone if it is subjected to rough handling during deployment or
retrieval, for example.

[0018]It is therefore desirable to have a vertical geophone arrangement in
which the bobbin and coil assembly is free to rotate within the geophone
case and in which the lower frequency spring that forms part of the
electrical circuit is not supported by a resilient contact spring.

[0019]3. Identification of Objects of the Invention

[0020]A primary object of the invention is to provide a geophone having a
3 dB increase in sensitivity over geophones of prior art.

[0021]Another object of the invention is to provide method and apparatus
for increasing geophone magnetic flux density by moving or changing pole
piece geometry.

[0022]Another object of the invention is to provide a method and apparatus
for precisely controlling geophone damping by tightly controlling the
overall mass of a geophone coil/bobbin assembly.

[0023]Another object of the invention is to provide a vertical geophone
characterized by lower distortion of the natural sinusoidal response to
an impulse source.

[0024]Another object of the invention is to provide a vertical geophone
having a frequency spring that is disposed directly on the lower end cap,
which also forms part of the electrical circuit.

SUMMARY OF THE INVENTION

[0025]The objects described above and other advantages and features of the
invention are incorporated in a geophone that is characterized by a
parasitic flux leakage that is significantly reduced lengthening the
spacing between the magnetic pole pieces. The result of moving the pole
pieces further away from the magnetic center is a shift in the magnetic
flux towards and beyond the ends of the magnet. The axial length of the
foil bobbin and the outer cylindrical housing are likewise increased, and
the positions of the upper and lower coils are moved towards the ends of
the magnet as appropriate to align with the shifted radial magnetic flux.

[0026]In a preferred embodiment, the geophone employs an Alnico-9 magnet.
The thickness of the pole pieces is increased while the effective length
of the pole pieces is decreased, as compared to Alnico-9 geophones of
prior art. The wall thickness of the cylindrical housing is also
increased minimize flux leakage outside of the housing due to the
increased flux density.

[0027]The coil bobbin ideally includes a provision for receiving a third
coil winding between the upper and lower coils. This third coil is a mass
tuning coil whose purpose is to adjust the overall mass of the bobbin
assembly with greater accuracy and precision than can be achieved by
machining alone. Mass is adjusted by adding or subtracting one or more
turns of wire in the tuning coil. The tuning coil is preferably
electrically shorted for increasing geophone damping.

[0028]The geophone according to an embodiment of the invention is a
vertical geophone that includes a lower frequency spring which is
positioned directly on the lower end cap. This arrangement eliminates the
"spring supported by a spring" arrangement of prior art geophones to
minimize geophone distortion and simplify tuning of the frequency
springs. A contact spring is positioned between the lower frequency
spring and the lower pole piece for forming part of the geophone
electrical circuit. One surface of the contact spring includes a
plurality of wiper surfaces that ensure consistent sliding electrical
contact against either the bottom surface of the lower pole piece or the
upper surface of the lower frequency spring. The obverse surface of the
contact spring is preferably spot welded to the upper surface of the
lower frequency spring or the bottom surface of the lower pole piece,
respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]The invention is described in detail hereinafter on the basis of the
embodiments represented in the accompanying figures, in which:

[0030]FIG. 1 is a cross-section side view of a typical vertical geophone
assembly of prior art taken along the geophone longitudinal axis, showing
a magnet, upper, lower pole pieces, an outer cylindrical housing, and a
bobbin carrying electrical coils that is suspended within the cylindrical
housing between upper and lower springs;

[0031]FIG. 2 is an enlarged side view in partial cross-section of a
portion of the prior art geophone of FIG. 1, shown without the bobbin and
coils for simplicity;

[0032]FIG. 3 is a cross-section side view of a vertical geophone assembly
according to a first embodiment of the invention, showing upper and lower
pole pieces that have been extended axially away from the magnet compared
to the prior art geophone of FIG. 1;

[0033]FIG. 4 is an enlarged side view in partial cross-section of a
portion of the geophone of FIG. 3 shown without the bobbin and coils for
simplicity;

[0034]FIG. 5 is a side-by-side comparison of prior art geophone of FIG. 1
and the geophone of FIG. 3 according to a first embodiment of the
invention and a graph of radial magnetic flux in the air gap passing
between the pole pieces and outer cylindrical housing versus geophone
axial position for each geophone;

[0035]FIG. 6 is a detailed cross-section side view of a vertical geophone
assembly of FIG. 3 illustrating preferred geometrical ratios and shapes;
and

[0036]FIG. 7 is an enlarge perspective view of a contact spring for the
vertical geophone of FIG. 3 according to a preferred embodiment of the
invention, showing wiper contact surfaces formed therein for maintaining
consistent sliding electrical contact between the contact washer and an
adjacent member.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[0037]FIG. 3 illustrates an improved geophone 110 according to an
embodiment of the invention, which for a given magnet type and geometry
may have a greater sensitivity than geophone 10 (FIG. 1) of prior art.
FIG. 3 is drawn to the same scale as FIG. 1. FIG. 4 is a cut away view in
partial cross-section of geophone 110 of FIG. 3, shown without bobbin 130
and coils 122, 124 for simplicity. FIG. 5 is a side-by-side comparison of
geophone 110 with prior art geophone 10. On the left side FIG. 5, a cross
section of the typical geophone 10 of FIG. 1 is shown. On the right side,
a geophone 110 of FIG. 3 and according to an embodiment of the invention
is shown. The two geophone drawings 10, 110 are illustrated in the same
scale and positioned along a common centerline so that a comparison may
be readily made.

[0038]Referring to FIGS. 3-5, as compared to prior art geophone 10 of FIG.
1, geophone 110 is characterized by a parasitic flux leakage that is
significantly reduced, because the lg dimension is increased. Magnet
114 is the exact same size as magnet 14, yet geophone 110 has a longer
axial length than geophone 10. The upper and lower pole pieces 116, 118
have been extended further beyond the ends of magnet 114. The result of
moving the pole pieces further away from the magnetic center is a shift
in the magnetic flux towards and beyond the ends of magnet 114. The axial
length of bobbin 130 and outer cylindrical housing 120 are likewise
increased, and the positions of upper and lower coils 140, 142 are moved
towards the ends of magnet 114 appropriately to align with the radial
magnetic flux. Because the length lg of the parasitic air gap 125
between the upper and lower pole pieces 116, 118 is extended, a larger
parasitic air gap reluctance Rp is created, and less flux leakage
occurs.

[0039]The lp dimension of geophone 110 may be less than, equal to or
greater than lp of geophone 10. For a given lg a greater
lp results in greater sensitivity and greater linearity, but at the
expense of a greater lpp and greater weight, size and deployment
costs.

[0040]Parasitic flux leakage is shown by flux lines 174 (FIG. 4). This
parasitic flux leakage may be modeled as a parasitic reluctance Rp
in parallel with the magnetic circuit formed by series combination of
reluctances of the upper and lower air gaps and the cylindrical outer
housing, combined with the counter magnetic motive force induced in coils
122, 124 as the coils cut the lines of flux. In practice, modeling is
more difficult as the overall magnetic field properties are highly
dependent on the material and geometry of magnet 114, including the
lm/dm ratio.

[0041]The reluctance Ra of one of the upper or lower annular air gaps
is approximated by

R a ≈ r a π d p l p μ o ( 1
) ##EQU00001##

where ra is the radial distance between the upper or lower pole piece
and the outer cylindrical housing 120, dp is the outer diameter of
the upper and lower pole pieces, lp is the length of the upper or
lower pole piece, and μo is the permeability of free space. The
parasitic reluctance Rp is approximated by

R p ≈ l g π d p t l μ o ( 2
) ##EQU00002##

where lg is the longitudinal distance between the lips of the upper
and lower pole pieces, dp is the diameter of the pole piece, tl
is the thickness of the pole piece lip, and μo is the
permeability of free space.

[0042]Flux leakage is minimized by decreasing the annular air gap
reluctance Ra and increasing the parasitic reluctance Rp. Thus,
the greater the ratio of Rp/Ra, the greater the geophone
sensitivity will be. From the simplified relations of equations (1) and
(2), it can be shown that

R p / R a ≈ l p l g r a t l . (
3 ) ##EQU00003##

According to a preferred embodiment of the invention, magnet 114 is an
Alnico-9 cylinder with a length-to-diameter ratio (lm/dm)
between 0.5 and 1.0, and the geophone ratio

l p l g r a t l ##EQU00004##

is greater than 14. More preferably still, lm/dm ranges between
0.7 and 0.9, and

l p l g r a t l ##EQU00005##

is greater than 20.

[0043]Geophone performance can also be considered using other geometric
ratios. For example, the smaller the lg dimension relative to the
ra dimension, the greater the geophone sensitivity will be. Prior
art geophones 10 typically have a lg/ra ratio under 2.5,
whereas geophone 110 has a lg/ra ratio greater than 4, and more
preferably still, greater than 6.0. Likewise, for a given pole-to-pole
distance lpp, the greater the lg dimension (at least until
lp approaches tl), the greater the geophone sensitivity will
be. Prior art geophones 10 typically have a lg/lpp ratio less
than 0.25, whereas geophone 110 has a lg/lpp ratio greater than
0.4, and more preferably still, greater than 0.5.

[0044]The dimensions of upper and lower pole pieces 116, 118 is also
important to the functioning of geophone 110. The ratio of the pole
length lp to the lip length ll is related to the thickness
tp of the pole piece. It the pole pieces are too thin, too much flux
will leak beyond the top and bottom ends of the upper and lower pole
pieces 116, 118, respectively. Conversely, if the pole pieces are too
thick, the geophone 110 becomes too heavy to be commercially attractive.
Preferably, lp/ll ranges between 0.2 and 6.0, and more
preferably still, between 0.4 and 1.0.

[0045]Similarly, in a typical geophone 10 of prior art, the outer
cylindrical housing 20 is made quite thin to minimize weight. The wall
thickness of th of housing 20 is typically about one-half the radial
air gap distance ra. However, in geophone 110 according to a
preferred embodiment, the wall thickness of cylindrical housing 120 is
greater to minimize flux leakage outside of the housing. Ideally, the
th/ra ratio exceeds 0.7, and more ideally still, 1.0. As shown
in FIG. 6, this increased wall thickness occurs only in the region in
which the active magnetic circuit region--the region which is axially
located between the top of upper pole piece 116 and the bottom of lower
pole piece 118. The top and bottom regions of housing 120 that extend
beyond the pole-to-pole longitudinal region are have a reduced wall
thickness to minimize weight.

[0046]Geophone 110 is also preferably characterized by a tp/lm
ratio greater than 0.15, a lg/ra ratio greater than 5.5, a
thra ratio greater than 0.7, a lpll ratio greater
than 1.7 and less than 5.0, a dp/dm ratio greater than 1.11 and
less than 1.14, a ll/lm ratio greater than 1.11 and less than
1.14, and a ra/dm ratio greater than 0.097 and less than 0.12.

[0047]The graph of FIG. 5 illustrates the annular flux distribution of
geophone 110 according to the preferred embodiment compared to a typical
geophone 10 of prior art. Geophone 110 results in a substantial increase
in flux density 174 over flux density 70 from geophone 10. The positions
at which the flux density is greatest is also extend toward the
longitudinal ends of magnet 114. This increased flux density 174 results
in a sensitivity improvement of a 3 dB or more of geophone 110 over the
prior art geophones 10.

[0048]Referring to FIGS. 3 and 6, bobbin 130 includes a provision for
receiving a third coil winding 150 between upper coil 140 and lower coil
142. Coil 150 is a mass tuning coil whose purpose is to adjust the
overall mass of bobbin 130 with greater accuracy and precision than can
be achieved by machining alone. Mass is adjusted by adding or subtracting
one or more turns of wire in coil 150. Coil 150 may be an open-circuited
or short-circuited coil, but a shorted coil 150 results in induced
currents, counter magnetic motive force and increased geophone damping
compared to an open-circuited coil. U.S. Pat. No. 4,159,464 issued to
Hall, Jr. on Jun. 26, 1979 discloses a similar arrangement of s geophone
with a mass tuning coil, and it is incorporated herein by reference.
However, Hall Jr. teaches away from the preferred embodiment of short
circuiting the mass tuning coil 150.

[0049]Referring back to FIG. 3, according to a preferred embodiment of the
invention, lower frequency spring 134 is positioned directly on lower end
cap 126, thus alleviating the difficulties arising from the "spring
supported by a spring" arrangement of prior art geophone 10. This
arrangement minimizes distortion of the natural geophone sinusoidal
response to an impulse vibration. The lower lead 182 of lower coil 142 is
soldered to the outer circumference of lower frequency spring 134, as
before. Electrical contact between the inner circumference of lower
frequency spring 134 and the lower surface of lower pole piece 118 is
bridged by a contact spring 136 that is positioned therebetween.

[0050]The upper surface of contact spring 136 includes a plurality of
wiper surfaces that ensure consistent electrical contact against the
bottom of lower pole piece 118. The lower surface of contact spring 136
is preferably spot welded to the upper surface of lower frequency spring
134 to eliminate an additional sliding electrical contact there.
Alternatively, the lower surface of contact spring 136 includes a
plurality of wiper surfaces that abut the upper surface of lower
frequency spring 134, and the upper surface of contact spring 136 is spot
welded to the bottom of lower pole piece 118. In this manner, contact
spring 136 is free to rotate with respect to lower frequency spring 134.

[0051]FIG. 7 illustrates a preferred embodiment of contract spring 136
according to the invention. Contact spring 136 is preferably a
washer-like structure that has a first surface 193 that includes a
plurality of wipers 195, i.e., movable electrical contact surfaces or
edges, for ensuring consistent electrical contact between contact spring
136 and either the lower surface of lower pole piece 118 or the upper
surface of lower frequency spring 134 (FIG. 3). Wipers 195 may be formed
by bending the washer-like body of contact spring 136 or by punching
portions of contact ring 136 by use of a die, for example. The wipers are
formed to be resilient springs so that they are under compression when in
stalled in geophone 110, thus maintaining contact with the adjacent
member. The obverse surface 197 of contact spring 136 is preferably spot
welded to lower frequency spring 134 or lower pole piece 118,
respectively (FIG. 3), so that only one sliding electrical interface need
exist between lower frequency spring 134 and lower pole piece 118.

[0052]The Abstract of the disclosure is written solely for providing the
United States Patent and Trademark Office and the public at large with a
way by which to determine quickly from a cursory reading the nature and
gist of the technical disclosure, and it represents solely a preferred
embodiment and is not indicative of the nature of the invention as a
whole.

[0053]While some embodiments of the invention have been illustrated in
detail, the invention is not limited to the embodiments shown;
modifications and adaptations of the above embodiment may occur to those
skilled in the art. Such modifications and adaptations are in the spirit
and scope of the invention as set forth herein: